1. Technical Field
The present invention relates to a device for use in a borehole to place a radioactive source in one of the geological formations through which the borehole passes. Such an operation is a preliminary operation prior to measuring formation subsidence. Sources put into place in this way serve as tracers whose depths are measured by means of radiation detectors lowered down the borehole. Subsidence is determined by evaluating the variation over time of the depths of the sources.
2. Background Information
A source is conventionally put into place by firing a radioactive bullet, i.e. a bullet containing a radioactive source, into the formation using an explosive device (referred to as a "gun" by the person skilled in the art) suitable for being moved along a borehole.
For the source to be put into place in satisfactory manner, the distance by which it penetrates into the formation from the borehole must lie within a determined range. If it does not penetrate far enough, there is a danger of the radioactive bullet falling into the borehole and contaminating the fluids therein. Conversely, if it penetrates too far, it may be impossible to detect the radioactivity coming from the bullet by means of a detector lowered down the borehole, particularly if the formation has a high level of natural radioactivity.
Naturally, penetration depends on the hardness of the formation into which the shot is fired. The problem is particularly difficult in very soft formations, such as chalk. The penetration distance can be varied by varying the amount of explosive, however to limit penetration adequately in a very soft formation it would be necessary to reduce the amount of explosive to such an extent that bullet firing is no longer certain.
The invention seeks to place a radioactive bullet at a suitable penetration distance, even in very soft formations.